A 9-year-old girl suffered from severe dwarfism, mental retardation, and a ring chromosome No. 1. She developed anemia characterized by multinuclearity of the marrow erythroblasts, and finally died with acute myeloid leukemia. The leukemic cells had a distinctive, pseudodiploid karyotype lacking the ring chromosome. The clinical features are compared with those of two other children with ring chromosome No. 1. The hematological abnormalities are presumed to be related to the development of the leukemic clone.